1. Field of the Invention
The present invention is directed to an implantable medical stimulation system of the type which is optionally operable in a bipolar mode or in a unipolar mode, and in particular to such a device having a connector adapted to receive a bipolar lead or a unipolar lead, depending upon the intended mode of operation.
2. Description of the Prior Art
Medical stimulation devices, such as heart pacemakers, are known in the art which can be optionally operated in a bipolar mode or in a unipolar mode. When employing a unipolar electrode, a body region of the patient in whom the system is implanted is connected by the unipolar electrode to a terminal within the stimulation device which carries the signal potential, so that electrical signals can be conveyed via the electrode to and from the body region. The reference potential is applied at a different location of the body of the patient by a separate means for applying the reference potential. A unipolar electrode therefore usually has only one conductor.
When a bipolar electrode is employed, both the terminal carrying the signal and the terminal carrying the reference potential, at the stimulation device, are connected by the bipolar electrode to a region of the body to which and from which electrical signals are conveyed. A bipolar electrode therefore contains two conductors, each of which terminate in exposed contacts which are relatively close to each other, and which are adapted to make an electrical connection with the body region. It is thus possible to operate the device in a unipolar fashion using a bipolar electrode, by connecting only one conductor of the bipolar electrode to the terminal of the device which carries the signal potential. The reference potential is then applied to the body of the patient in some other way. It is, of course, not possible to operate the device in a bipolar fashion using a unipolar electrode.
Systems of the type described above are known in the art wherein the stimulation device is a heart pacemaker, and the electrodes lead from the heart pacemaker to the patient's heart. The pacemaker includes means for stimulating heart activity and means for detecting heart beats. Stimulation pulses from the means for stimulating are conducted to the heart via an electrode, and signals corresponding to the electrical activity of the heart are also conducted via the electrode to the means for detecting in the pacemaker. Such known pacemakers are implantable in the body of a patient with a unipolar or a bipolar endocardial electrode connected thereto at the time of implantation. The electrode is conducted through the vein system of the patient to the appropriate atrium or ventricle of the heart, and is anchored therein. Under some circumstances, it may be desirable after implantation of the system to switch from bipolar to unipolar operation and vice versa. Such switching can ensue telemetrically using an external device, known as a programmer. It is, of course, a prerequisite to such switching that the system be implanted with a bipolar electrode, if operation in the bipolar mode is desired as an option.
Switching from bipolar stimulation to unipolar stimulation, for example, may be desired because the amount of energy required for achieving a stimulation is lower in most patients for unipolar stimulation. Switching from unipolar to bipolar stimulation is necessary when undesirable stimulation side-effects, for example muscle tics, appear in the body region at which the reference potential is applied. In the case of a heart pacemaker, this will normally be in the region of the electrically conductive housing of the pacemaker, which is electrically connected to the terminal of the device carrying the reference potential. A switch from unipolar to bipolar detection can be useful if substantial noise is present given unipolar detection. Such noise is usually less pronounced given bipolar detection. Whereas switching from bipolar to unipolar operation does not pose significant problems, switching from unipolar to bipolar operation, particularly after implantation of the system has already been undertaken, makes it necessary that the attending physicial be sure that a bipolar electrode is in fact connected to the heart pacemaker. In the event of a mistake, proper stimulation is no longer insured, and thus life-threatening situations may arise for patients who are completely dependent on the artificial stimulation by the heart pacemaker, or are dependent thereon to a substantial degree.